Among powder metallurgical products, sinter-forged products, in particular, are used as members that are required to have especially high strength, such as connecting rods for automobile engines.
Iron-based alloy powders of an Fe—Cu—C type in which Cu powder and graphite powder are mixed with pure iron powder are commonly used as precursor powders for sinter-forged products (PTL 1 to 4). A machinability enhancer such as MnS may also be added to a precursor powder to enhance machinability (PTL 1, 4, and 5).
In recent years, there has been demand for even higher strength materials for connecting rod applications due to progress toward more compact and higher performance engines. Consequently, studies have been conducted in relation to optimization of the amounts of Cu and C (PTL 1, 2, and 5), but the effect of improving strength has been limited.
PTL 3 proposes using a pre-alloyed product obtained by pre-alloying an alloying element, such as Mo, Ni, or Cu, with iron powder. However, not only are alloying elements expensive, they also form hard structures such as martensite in an iron-based alloy powder. Consequently, a sintered body obtained using an iron-based alloy powder containing some alloying elements suffers from a problem of poorer machinability.
In response to this problem, PTL 4 proposes a technique by which the strength of a sintered body can be improved while maintaining machinability of the sintered body by only pre-alloying Cu with iron powder.